There has been an active effort to conserve water in desalination of seawater or salt water and an effluent recovery system by increasing the water recovery rate of an RO membrane system. Operating an RO membrane system at a high recovery rate involves the concentration of scale components, which cause scaling. Examples of the types of scale include calcium carbonate, calcium sulfate, calcium sulfite, calcium phosphate, calcium silicate, magnesium silicate, magnesium hydroxide, zinc phosphate, zinc hydroxide, and zinc carbonate basic. The formation of calcium carbonate scale during the high-recovery operation of an RO membrane system becomes significant particularly in an effluent recovery system, in which water having a high M alkalinity may be present.
Phosphorus-containing scale inhibitors, such as inorganic polyphosphates (e.g., sodium hexametaphosphate and sodium tripolyphosphate) and phosphonic acids (e.g., aminomethylphosphonic acid and phosphonobutanetricarboxylic acid), have been used for inhibiting the formation of calcium scale in an RO membrane treatment, because the phosphorus-containing scale inhibitors have a relatively low molecular weight and a large scale-inhibition effect.
There have been demands for phosphorus-free scale inhibitors due to the regulation of the phosphorus concentration in effluent.
PTL 1 proposes a phosphorus-free calcium carbonate scale inhibitor that is a copolymer of maleic acid and isobutylene or a terpolymer of maleic acid, vinylethyl acetate, and ethyl acrylate.
In PTL 1, a terpolymer that corresponds to the component (A) used in the present invention is described, and polyacrylic acid is described as an example of the other scale inhibitor that can be used in combination with the terpolymer. However, in PTL 1, no mention is made of a specific example in which the above scale inhibitors are used in combination, and only a general description is provided without any discussion of applicable conditions, advantageous effects, or the like.
PTL 2 proposes a technique in which an AA (acrylic acid)-AMPS (2-acrylamide-2-methylpropylsulfonic acid) copolymer and PMA (polymaleic acid) are used in combination.
In PTL 2, although it is described that using PMA and the AA-AMPS polymer in combination produces a large scale-inhibition effect, no mention is made of the component (A) used in the present invention.
In PTL 3, an example in which polymaleic acid (molecular weight: 580) and maleic acid-ethyl acrylate-vinyl acetate (molecular weight: 850) are used in combination is described. In PTL 3, the aqueous systems to which the above polymers may be applied are described as follows: “With respect to aqueous systems which may be treated according to the present invention, of particular interest are cooling water systems, steam generating systems, sea-water evaporators, reverse osmosis equipment, bottle washing plants, pulp and paper manufacturing equipment, sugar evaporator equipment, soil irrigation systems, hydrostatic cookers, gas scrubbing systems, closed circuit heating systems, aqueous-based refrigeration systems and oil production and drilling systems”. However, in PTL 3, the above polymers are not supposed to be applied to reverse osmosis membranes. PTL 3 does not disclose any substantial application of the polymers as a scale inhibitor for RO membranes. In PTL 3, no mention is made of the qualities of water that may be treated with an RO membrane or the specific conditions and composition of the polymers suitable for a specific RO membrane treatment.
PTL 1: WO2012/114953
PTL 2: JP 2013-531705 A
PTL 3: JP H2-280897 A